A problem of considerable concern in some IPT Systems, such as IPT roadway applications, is suppressing the magnetic flux that leaks out of the air gap in such systems, or which leaks out of a region where magnetic flux is required for inductive power transfer. This problem exists for charging and powering of stationary and moving systems, whether they are high or low power systems. Examples include: low power electronic devices such as cell phones, tablet computers, notebook computers computer peripherals, lighting; higher power devices such as domestic appliances, robots, automatically guided vehicles, as electric vehicles (EVs), industrial equipment.
The problem can be described using the example of a system which enables EV wireless charging from a roadway. In such a system the roadway includes a flux coupler which can be energised by a power supply to provide a time varying magnetic flux. A corresponding flux coupler on the vehicle allows the vehicle to be wirelessly (inductively) coupled to the power supply. There is an air gap between the flux coupling structures. The air gap may vary and be relatively very large and there may be considerable misalignment between the roadway flux coupler and the flux coupler on the vehicle. Magnetic flux which is not present in an appropriate region for coupling (referred to herein as coupling flux) is unwanted and is referred to herein as leakage flux. Leakage flux may escape from the gap between the flux coupling structures and interfere with other objects or systems. In this example the allowable leakage flux is required to be maintained at a very low level despite the relatively large amounts of power being transferred from the power supply to the vehicle.
The actual flux density allowed in the immediate vicinity of the vehicle may be 17 μT which may be compared with the earth's magnetic field of approximately 60 μT, the major difference being that the earth's field is a constant DC field while the IPT system will have an alternating field at a frequency between 10 and 140 kHz.
It is an object of the invention to reduce magnetic field that leaks out of a region where magnetic flux is required for inductive power transfer, such as the air gap in IPT systems.